Method of and apparatus for heat treatment



Aug. 1s, 1925.

JNVENTOR. z@ 4. m

ATTORNEY.

2 Sheets-Sheet l fwd @w 1 A u J C'. A. MARTIN METHOD OF AND APPARATUS FOR HEAT TREATMENT Filed Jan. 8. 1924 fw Aw.

TIME

TIME

TIME

Aug'. 18, 1925.

C. A. MARTIN METHOD OF AND APPARATUS FOR HEAT TREATMENT Filed kan. s, 1924 2 Sheets-Sheet 2 INVEVNTOR.-

` @gwn/g Patented Aug. 18, 1925.

UNITED STATES rPATENT oFFlcE-f.' A

vCHARLES A. MARTIN, Oll'sA CHICAGO, ILLINOIS, ASSIGNOR TO LEEDS & NORTHEUP-COM- l PAN Y, F PHILADELPHIA, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.

METHOD OI AND APPARATUS FOR HEAT TREATMENT.

Application led January' 8, 1924. Serial No. 684,946.

To all 'whom it may concern:

. Be it known that I, CHARLES A. MARTIN,

a citizen of the United States, residing in Chicago in the county of Cook, State of u Illinois, have invented a new and useful Method of and'Apparatus for Heat Treatment of which the fol/lowingis a specification.

My invention relates to a method of and apparatus for accentuating or rendering more easily observable chan es in the rate of change of temperature, o heat transfer or of an effect dependent upon or related to change of temperature orheat transfer;

and more particularly, my invention relates to a method of and apparatus for accentuating or rendering more easily observable the occurrence or time of occurrenceof a-critical or transformation poi-nt or points or o f the dec-alescence, recalescence or similar sta-ge in the heat treatment of material, particularly steel or alloy'steel, and more particularlyalloy steel or low .carbon steel whose critical or transformation points are '26 not sharply defined.

My invention relates more particularly to a method of and apparatus for enhancing the utility of a method of the character disclosed in Letters Patent No. 1,188,128, granted June 20, 1916 to W. J. Wrighton.

In accordance with my invention, a critical on transformation point is determined by an abrup-t'change in the rate of change of the difference between temperatures at points or regions at different distances from,

but preferably adjacent to, the mass undergoing heat treatmenty and whose rate .of

change of temperature effects the rate oftemperature change at at least one of the points or regions. This may be accom-' plished in any suitable way, but preferably by temperature-iesponsive vdevices located at the different points or regions and producing ellects which are combined to p-ro-` duce a jointefect whoserate of change corresponds with or depends upon the rate of l vchange of the difference between the temperatures at the different points or regions. Changes in the rate of change vof the Ytemperature difference, or in the rate of change of the yjoint effect, are preferably'magniied, f as by employing at one of the points or regions temperature? responsive devices greater in number than employed at the 55 [other point or region and cumulating the effects of al1 upon an indicating instrument, whereby the indication effected is amplified.

My invention resides in the method and apparatus of the character hereinafter described and claimed.

'For an understand-ing of my method, and for an illustration 'of some of the various forms my apparatus may take, reference is to be had tothe accompanying drawings, in which: f

Fig. 1 is a curve in the nature of a timetemperature curve, whereof the inliections at the beginning and end of a decalescence stage are n'ot sharply defined.

Fig. 2 is a similar curve whereof the beginning and end of a decalescen'ce stage are more sharply defined and joined by a portion of the curve resembling zero or at most slight increase in temperature with time.

Fig. 3 is a similar' curve in which the points rep-resenting the beginning and end ofv a decalescence stage are even morev sharply defined at the ends of a portion of the curve resembling actual decrease in temperature with time. I

`Fig. |4 is a vertical sectional view, parts in elevation, of a heat-treating'furnace with my invention applied thereto.

Fig. 5 is a plan view, on enlarged scale, partly in horizontal section of part of the structure shown in Fig. 4. v

Fig. 6 is a vertical sectional view, partly in elevation', of a temperature-responsive device comprising a multiple thermo-couple with a baffle of modified structure.

Fig. 7 is a diagrammatic view of one of various circuit arrangements in which the multiple thermo-couple is utilizable iny ac` cordance with my invention.

Fig. 8 isa diagrammatic view of lone of various circuit arrangements in which another form of my temperature-responsive device is utilizable.

`While my invention is, applicable in general for accentuating or rendering more easily discernible or observable changes in or the time of occurrence of change in rate of change of temperature, heat transfer or re lated effects, I'shall for purposes of illus# vtration describe its preferred field of application, to wit, in the practice of the method disclosed in theaforesaidLetters Patent No. 1,188,128, whereof it ischaracteristic that the time of occurrence of a more or less abrupt change in the rate of change of temn il.'

perature or heat transfer at the beginning or endv of a decalescence or recalescence stage 1n the heat treatment of steel or the like is determined Without regard to the actual or absolute temperaturesassumed by the steel or the surrounding medium b observmg the change in rate of change o the effect produced by a temperature-responsive device dis osed adjacent the steel under treatment.

llt has been found by me that the time of occurrence of the critical or transformation point `or points or beginning and end of a decalescence or recalescence period or stage may be more sharply dened or rendered more easily observable by utilizing a multiple or composite temperature-responsive device, different elements or components of .which are at diilerent distances from or differently located with respect to the steel undergoing heat treatment; or, in general, so located with res ect to each other that the rate of change o the difference between the temperatures they assume is small, negligible 0r zero until a critical point is reached when the rate of change of the diierence between the temperatures markedly changesor even reverses in sense. Each componentof the composite temperature-responsive device produces an efect whose magnitude 1s dependent upon the magnitude of the temperature, which may remain unknown, to which it is subjected, and the eiects of the several components are combined to produce a joint effect whose rate of change markedly changes in accordance with a marked change at the time of occurrence of a critical point in the rate of change of the difference between the temperatures to which the components are respectively subjected. The composite temperature-responsive device behaves as a slmple temperature-responslve device during the substantially regular change of temperature of the steel, but at the times of occurrence of the critical or transformation points and during the decalescence and recalescence stages behaves as a series of temperature-responsive devices Whose effects are cumulative, and therefore greater than the effect of the single temperature-responsive device, thereby increasing or magnifying the efects of absorption or loss of heat by the steel with change in the rate of change of temperature thereof characteristic of the. decalescence and recalescence stages.

My invention accordin 1y renders more useful the method of saidetters Patent No. 1,188,128 in general, and is of particular advantage when the steel under treatment is of a character Whose critical or transformation points are not sharply defined, as in the case of low carbon steels or alloy steels. My invention is of particular advantage also in cases where the mass or weight of the material under heat treatment is small,

.fined, while wit since the critical points obtained by my composite or multiple temperature-responsive device are more pronounced and sharply dea simple temperature-responsive device the critical points of'a small piece or mass of steel are not clearly dened. For example, when a simple or single temperature-responsive device is employed, as in the method of said Letters Patent No. 1,188,128, the smallest pieces of steel or the like whose transformation points can readily be determined are of the order of five or six ounces in weight; while by my method there are readily determinable the times of occurrence of the critical points of pieces of the order of one-half or one-quarter cu. in., about three or about one and one-half ounces, or for example, a pieceof drill rod one-quarter inch 1n diameter and two inches in length. v My invention is of particular utilit therefore, when the article undergoing eat treatment is not only of small size or small weight, but tI more so when in addition it is composed of alloy steel or low carbon steel. v Referring to Fig. 1, the curve A is of the nature of a time-temperature curve whose abscissae represent time and whose ordinates are electro-motive-forces, resistances or kindred eects dependent upon the temperature of a thermo-couple, resistance or equivalent temperature-responsive device disposed at the surface of or near the steel undergoing heat treatment. The actual or absolute temperatures of the curve A may be unknown, but nevertheless, as described in the aforesaid Letters Patent, the times of occurrence of the critical or transform tion points a and b are determined and are observable, the portion c of the curve Abetween the points a and b corresponding with the decalescence sta e or period.

The curve A of 1g. 2 illustrates the results obtainable by my invention, to wit, the accentuation or increase in the apparent, though not true, rate of change of temperature of the steel at the begmning or end of or during the decalescence sta e. As indicated at the points ai and b, t e changes in direction or inflections4 of the curve A are more marked, and therefore the times of occurrence `of the points a and b are more readily discernible. In this example, during the decalescence period c the apparent changes in temperature of the steel is zero, or there is at most a slight increase in temperature with time.

As indicated in Fig` 3, the time of oc'- currence of the decalescence stage is even more markedly accentuated, the apparent changes in the rate of change of temperature at the points aand b being greater than in Fig. 2, and during the decalescence period c the apparent rate of change of the steel is actually negative, that is, the temperaposite or multiple temperature-responsive device of any suitable character, several of which are hereinafter described.

Referring to Fig. 4, F isa furnace or equivalent heater Whose heating chamber,.

in the example illustrated, is the chamber within the member H in which heating is effected in any suitable way, as by combustion of oil, gas or otherfuel, but preferably, as in thel example illustrated, by heat evolved by the resistance conductor R, through which electric current of'suitable magnitude is passed to effect substantially uniform or at least regular rise in temperature of the article S undergoing heat treatment, and which in the example illustrated is a steel die, but which obviously may be any other article of allow steel, high or low `carbon steel, or any other equivalent material which exhibits one or more critical or transformation points, the time of occurrence of which is to be determined. The article S, which represents generically either a single article or a group of articles, is

supported in the heating chamber in any suitable way or by any -suitable means, in the example illustrated by the supporting wire or chain d'hung upon the arm e.

Disposed at the surface of or adjacent the article S is the composite temperatureresponsive device D, in this case being of the thermo-electric type comprising ay plurality of thermo-couples in series with each other. In the example illustrated, the device D comprises three thermo-couples, three of whose thermo-junctions f, f, f may for convenience be termed the hot junctions,

-two of whose thermo-junctions g, g may be for convenience termed cold junctions, and whose third cold junction is remote, preferably outside of the furnace, at any suitable place, as, for example, either at i, at the switch s, at the galvanometer G, or at the instrument or recorder I, Ias may be suitable or desirable. Each couple comprises dissimilar metals, as well understood in the art, and in the example illustrated, the couples are' formed of the wires 7c and m, of dissimilar metals, preferably large enough to be self-sustaining or supporting, and welded, soldered or otherwise joined to each other at the junctions f and The wire c to t-he right and the wire m to the left are preferably continued downwardly and extend through the plug or removable furnace bushing la., of lava or ,equivalent heatresisting non-conducting material' pierced with holes to accommodate the wires k and m, which at their lower or outer ends may be connected by conductors, copper or slmilar conductors to the associated instrument,

in which case the cold junction's at t at the ends of the wires m and k, where they 'emerge from the plug z, which carries the wires and the device D, forming therewith a unit. As well understood in the art, the connections from the lower ends of the wires lc and m may be made by conductors of the same materials as the wires lc and m to the switch s or beyond to either ofthe intion is either at the switch s or either of the instruments G or I.

The junctions f and g are adjacent each other and in practice may be a distance apart of the order of several inches, for example, two inches. Surroundin the thermo-couple wires are tubes or bushings fn, of clay or other suitable insulating material capable of withstanding high temperatures, the members -n serving to prevent contact between adjacent couple wires lc and m.

Upon a support o, of any suitable character, is disposed the battle member lc, having a. slot j through which the multiple thermo-couple or device D extends, whereby as between the hot junctions f and cold junctions g there is introduced a heat-baffling eii'ect or any equivalent effect for causing 'or allowing the hot and cold junctions f and g to assume somewhat different teinperatures or to change in temperature at different rates, particularly when the piece S is passing through a transformation stage, as, for example, the decalescence stage c of Figs. 2 and 3. The baiiie K may, as indicated on larger scale in plan kView in Fig. 5 be in the form of a disk. It may be composed of any suitable material, as clay or the like capable of `withstanding the temperatures involved.

As'indicated in Fig. 6, the baiile member K may be formed as a unit with the temperature-responsive device D, and therefore comprised also in the unit including the supporting plug t indicated inV Figs. 4, 7 and 8. In the arrangement of Fig. 6 there is integral with the member K the extension p pierced with holes through which extend the thermo-couple wires lc and m, which are spaced and insulated from each other by the member p.

Reverting to Fig. 4, upon throwing the uniformly, or at least regularly, as in dicated, for exam le,l in the left hand portion of the curve A, ig. 2, voltage readings may be taken-at intervals and thereby the curve A plotted as the readings progress. When the piece S reaches a temperature at whlch transformation begins, the point a of Fig,l

2 will have been reached. Then continuing to take readings of the galvanometer` G and plotting them against time, the portion c of the curve A will be derived, the time of occurrence of the sharply defined or pronounced inflection at a indicating to the reader of the instrument G that the piece S has reached the transformation point. Again, upon continuing readings and plotting them against time, the deflections abruptly increase, as at the point 6, whereupon the obse-rver knows that the piece S has passed through fthe transformation period and is again rising in tem erature at higher rate than during lthe erio c. Accordingly, by simply taking rea ings of the instrument G and plotting them against time, the .time of occurrence of the critical points is determined.

In practice, however, it is generally preferable and more usual to employ a curve drawing recorder, as I, of any suitable character, such, for example, as disclosed in Leeds Patent 1,125,699, involving a potentiometer with which is associated a galvanometer, in series with which through the switch s is the multiple thermo-couple D. A suitable arrangement of this character is indicated in Fig. 7, where the potentiometer comprises the source of current or battery B delivering current through the resistance R1, the magnitude of the current being suitably adjusted by employing, if desirable, a second resistance R2. `The one terminal of the multiple thermo-couple is connectedA Vthrough the galvanomete G to one terminal of the resistance R1, and the other terminal of the multiple thermo-couple is connected to the contact u adjustable along the resistance R1 to such osition that lthe fall of potential between rt e contact u and the left end of the resistance R1 is equal and opposite to the electro-motive-force produced by the composite thermo-couple, in which case the deflection of the galvanometer G is zero. The needle gef fthe galvanometer G controls the recorder mechanism, which effects adjustment of the contact u along the resistance 'R1 and produces a record.

The marker r of the recorder draws upon the recorder sheet or paper t, moving in the direction of the arrow, the curve A, on which are readily discernible the inflection points a: and b and the intervening portion c corresponding to the decalescence stage of the piece under treatment.

In kind, the abovedescribed operation is the same as described in the aforesaid Letters Patent No. 1,188,128. However, as ex- 'plained in connection with Figs. 2' and 3,.

the critical points and decalescence stage are more clearly and sharply defined or much more pronounced, and therefore more readil discernible, v particularly by' less skille furnace operators or heat treaters.

During the period of heat treatment in advance of the point on the curve A, Fig. 2, for example, the rise in temperature within the heating chamber is substantially uniform or at least regular, and suitably slow, with the result that the temperatures of the hot junctions f and cold junctions g are the same, or nearly so, and in any event, so nearly so that the junctions g are durin this eriod substantially ineffective as co1 junctlons, with the result that the electromotive-force produced by the multiple couple is practically that due to a single couple whose hot junction is within the furnace and whose cold junction is at i, s, G or I, external to the furnace. In other words, since the temperature of the couples g is the same or substantially the same as the temperature of the hot junctions f, the situation is the same, as regards two of the three couples of the composite temperature-responsive device, as if they were absent or were p ucing no substantial electro-motive-forces because the two junctions of each are at substantially the same temperature.

However, when the piece S enters into the i transformation period, it continues to absorb heat without rise in temperature, or at least with decreased rate in rise in temperature notwithstanding the continuance of delivery of heat at substantially the previous rate into the heating chamber, with the result that the hot junctions f, closer to the piece S than the junctions g, either discontinue rising in temperature,or at any rate, continue to rise in temperature 'at a lower rate than before, while the junctions g continue to rise in temperature at a different or higher rate than the junctions f, with the result that the total electro-motive-force produced by the three thermo-couples in series with each othereither continues to increase at a slower rate, remains of fixed magnitude as indicated during the periode of Fig. 2, or actually decreases -with time, as indicated by the period c of Fig. 3. In any event, the point a of the curve becomes more pronounced than in the case of Fig. 1, y

point continuing to rise in temperature at ysubstantially equal rates,

though their actual temperatures may be somewhat dif.-

ferent, as in the first portion of the curve A before reaching the oint a.

The presence of a y aiile member K, which may or may not be used, ac centuates the j difference in rate of rise or change of temat the same temperature or at a pre etermined small' difference intemperature during theinitial portion of the heating period v before the point a is reached, thejmultiple couple in effect operates Lsubstantially as a v single couple as regardsthe electro-motive- .force produced;y yet when the transforma- `tion period is reached, due to the` difference in distance between the` hot and cold junctions f tions from the --piece` S, lthe difference in temperature between the vhot .and cold juncandg increases or becomes more marked, 'though both yhotandl cold junctions l vmay be atthe timeincreasing in temperature, lwith the result that the rate of change 1 4of, the. electro-rnotive-force impressed upon the reading instrument or recorder is to" y greater extent changed at theinflection f points a and b than inf the case a single or simple thermo-couple were employed, thereby effecting an amplification o`r magnification ofthe changey in the rate 'of change of A4temperatur-cof the piece. v

Since the multil le thermo-couple is of such structure an' lemployed under such conditions that its total lelectro-motive-force is not materiallydiHeren-t from or is of the same order vof' magnitude of `that of-a single 'thermo-couple, the scale '-of the re- A,

corder chart orfpaper.y t, measured, trans- 1 versely thereofvat'right'angles to its direction of movement,'-isfpractically.'the same' Y as in the case of .use of a singlefor simple 'thermo-couple. Whilethe recorderfpaper t. neeff not be graduatedtranverselygin "tem:

" "Avrperatuite's or electro-niotive-'forcefor equivalent units, it may .befifsuitable.Soridesirable.A A feature of'advfantageand impor- L 'tance is that lthe temperatures or .electrocouple is employed will `not `differ greatly motive-forces indicated by :the recorder upon its ychart or papert when'aJ single thermothe multiple couple are disposed adjacent',l

theJ hot junctions, with the result that the magnitude: of the electro-motive-forces effective upon the indicating or recording instruments is of the same order of magnitude as that of a single thermo-couple, and yet makes possible accentuation and sharper definition of the inflection `points. The structure and mode of operation are therefore distlnguishable from the case where a multiple thermo-couple is employed with all of its cold junctions remote from its hot junctions, in Whlch case the electro-motive-force effective upon the indicating or recording instruments will simply be multiplied.

In advance of the point a, Fig. 2 for example, and after the point b, the difference between the temperatures at the hot junctions f and the cold junctions g is small or zero, or in any event, substantially constant, and the rateof 4change of the difference between these temperatures is substantially constant or Azero, and the multiple thermocouple is in effect substantially a simple or single thermo-couple having one hot junctron and vone remote cold junction; at and between the Kp oints a and b, however, lthe diii'erence between the temperatures at the .hotjunctions f and the cold junctions g becomes finite. or increases, or reverses in Sense, the rate of change of the temperature difference between junctions f and g markedly changes in magnitude, and the multiple thermo-couple now behaves as a plurality of couples serially related and having substantial difference of .temperature between its hot junctions fand cold junctions g, causing to be added to or subtracted from the jelectro-motive-force due to the single couple vwhose c'old ,junction isA remote or external to vthe furnace an electro-motive-force equal to the sum pf the electro-motive-forces of those the furnace,jthe greater will be that electromotive-force which' is added to or subtracted lfrom the electro-motive-force of the thermo- 'couples Whose coldjunctions are remote or external to; the furnace, and the more sharplywlll the critical points be defined. It will accordingly be understood that it is preferable to have but one cold junctionrelnoteor external to `the-furnace and to haveaslarge -number of component couples with' v their vcold junctions within the furnace adjacent butVA spaced from'the hot junctions f. While in theexample illustrated there is a total of three thermo-couples, one of whichjha's its cold junction remote or external to the furnace and two having their cold junctions at g within the furnace, lthe effects described may be further accentuated by increasing the number of cou les having their cold junctions g within t e furnace. It will be further understood that my invention is present if there are utilized but two thermocouples, having one cold junction at g within the furnace and the other cold junction remote or outside the furnace.

While the employment of a multiple thermo-couple is preferred, it shall be understood that the principle of my invention is inherent in the employment of composite or multiple temperature-responsive devices of other characters, whether or not electrical.

In lieu of employment of thermo-couples, the composite temperature-responsive device may, as indicated 1n Fig. 8, comprise the resistances M and N, havin substantial temperature co-eicients, pre erabl both positive, and either equal or unequa These resistances form with the plug h a removable and detachable unit which may include also a baille member K.. As indicated, the resistance M is placed nearer the piece S undergoing heat treatment than resistance N. These resistances may be used in any suitable circuit arrangement for making manifest-changes in resistance or effects dependant upon changes in resistance` The result will be in general the same as that hereinbefore described when using a multiple thermo-couple, to wit, the transformation points will be more sharply defined and more easily determinable as to time of occurrence. In the example illustrated, how-- ever, the resistances M and N are connected in two adjacent arms of a Wheatstone bridge in whose other arms are included the resistances R3 and R4. As indicated, the connections are effected with the resistances M and N by three conductors only, since one conductor connects with both resistances. The conductor which connects with both resistances may be one of the conjugate conductors of the bridge, in which is included the source of current or battery B, while in the other conjugate conductor is connected the galvanometer G, which may control a recorder mechanism, as in the aforesaid Leeds Patent No. 1,125,169, in which case the resistance R* is varied by the recorder to restore balance, and in restoringr balance produces a record.

Before the piece S reaches the transformation stage, the resistances M and N increase in temperature, and therefore in -resistance, regularly or substantially uniformly, or at any rate, at substantially equal rates, with the result that the unbalancing of the bridge and its re-balancing by the recorder progresses with regularity or substantial uniformity, thereby describing that portion of the curve A, Fig. 2, for example, to the left of the inflection point a. When the transformation point is reahed, however, the resistance N will continue to rise in temperature, while the resistance M will increase in temperature at a slower rate than before, with the result that the brid e will be sharply unbalanced, and to re-ba ance, the recorder marker w1ll move abruptly transversely of the recorder sheet, thereby sharply defining the critical or transformation point a. At the end of the transformation period the resistance M will risein temperature at higher rate than during the transform-ation period c, and there will result then the sharp inflection at the point b.

Here again the difference between the temperatures to which the resistances M and N are subjected is small, zero, or at least constant before the critical point a, and after the critical point b, and the change in the rate of change of the difference between these temperatures at the time of occurrence of the critical points is magnified or accentuated as to its effect upon the indicating or recording instruments, thereby rendering the critical points more easily observable.

It is characteristic of my invention in general, and of all the arrangements hereinbefore described, that there is produced at different localities around or adjacent the piece or pieces undergoing -heat treatment a plurality of effects whose magnitudes depend upon the temperatures Vat those localities, and those effects are combined to produce a joint effect in the rate of change of whose magnitude are caused accentuated and sharply defined abrupt changes at the times of occurrence of the critical points due to changes in the rate of change of the difference between the temperatures at the different localities at the times of occurrence of the critical points.

What I claim is: A

1. In the art ofheat-treating material, the method of determining when it passes through a. critical point, which comprises producing effects dependent upon the temperatures at different localities adjacent said material, combining said effects into a joint effect, and determining the time of occurrence of an abrupt change in the rate of change of the magnitude of said joint effect.

2. In the art of heat-treating material, thel method of determining when it passes through va critical point, which comprises producing effects dependent upon the temperatures at different localities adjacent said material within a medium surrounding said material and through which heat transfer is effected, the rate of change of temperature at at least one of said localities reecting the rate of change of tem erature of said material, combining said e ects into a joint effect, and determining the time of occurrence of an abrupt change in the rate of change of the magnitude of said joint effect.

3.` In the art of heat-treating materia-1,'the

l'nethod 'of dctermlning when it passes through avv critical point, which .comprises producing effects 'dependent upon the tem;

peratures at different'localities adjacent'said material, combining,r said effects into a joint leffect the rate of change of Whose magni l tude is dipendent upon' the `rate of change v Qfrlthe di at'said localities, and noting the timev .of ocerence between the temperatures n the' art of heat-treating material,

Ithe method of determining when it passes through a critical point, ,whichcomprises 'producing eEects' dependent upon the teini peratures at'different localities adjacent said said localities, and Anoting the; time of occur- I ,chan

` n '1aterial within a medium surrounding said material and through which heat transfer is effected, the rate' of change of temperature at at least one of said localities refiecting the rate of changeof temperature of said material, combining said effects into a joint effect thel rate of change' of whose magniture is dependent upon the-rate o f change'of the difference between the temperatures at rence;of an' abrupt changin the rate of n the .art o heat-treating material, the method of determining when it passes through' a critical point, which comprises determmmg the timeof voccurrence of an abrupt change in the rateof change of the difference between temperatures at different localities adjacent said material.

6, In the art of heat-treating material, the method of determining when it passes through .a critical point, which comprises determining the time of occurrence of an abrupt changefm the rate of change of the difference between temperatures at regions at `different distances from said material in the heat transfer medium surrounding said mater1al,.the location of at least one of said regions bemg so chosen that the temperature changes thereat are dependent upon the changes of temperature of said material.

7. In the art of heat-treating material, the method of determining when. it passesl through a critical point, which comprises l determining the time of occurrence of an abrupt change in the rate of change of thel difference between temperatures at regions vat different distances from said material in the heat transfer medium surrounding said material, said regions being so located with respect to said. material that the rates of change of tem eratures thereat are 'to different degrees ependent upon the rate of change of temperature of said material.

' 8.4 In the art of heat-.treating material, the

' method of detennining, when it passes through a .critical point, which. -comprises producingl effects dependent upon the teme of the ma nitude of Said joint effect.'

.peratures vat different localities adjacent said material, limpeding equalization of teniperatures at said localities, combining said effects into a joint effect, and noting the time of occurrence of an abrupt change 1n the rate of change of the magnitude of said material within a medium surrounding said material and through which heat transfer is effected, the rate of change of temperature at at least one of said localities refiectin .thel rate of change of temperature of sai material, impeding heat transfer -between said localities, combining said effects into a joint effect, and determining the time of occurrence of an abrupt change in the rate ofl changeof the magnitude of said j ointeffect.

"10. The method of determining the time ofl occurrence of a critical point of a material undergoing temperature change;

yof' effects dependent upon the temperature at a locality adjacent said first named locality but at a different distance from said material, and determining the time of occurrence of an abrupt change in the rate of change of magnitude of said combined effect` l 11. The method of determining the time of occurrence of a critical point of a material undergoing temperaturel change, which comprises producing an effect de- 'pendent upon the temperature of a locality adjacent said material, combining with said effect to produce a joint effect a multiple vof effects dependent upon the temperature at a locality adjacent said first named locality but at a different distance from said material, impeding'- equalization of temperature between said localities, and determining the time of occurrence of4 an abrupt Vchange in the rate of change of magnitude of said combined effect.

12. The method of determining when alloy steel or low carbon steel passes through a critical point, which comprises determining the time of occurrence of an abrupt change in the rate of change of the dierence between temperatures at different localities adjacent said steel.

13. The method of determining when alloy steel or low carbon steel passes throughl change of temperature at at least one of said 130 9o wh'ichcomprises producing an effect depend- .ent upon the temperature ofa locality ad- 'jace'ntsaid material, combining with said Veffect to produce a joint effect a. multiple 'localities refiecting the rate of change of small mass of steel of an order'of wei ht said steel.

,temperature of said steel, combining' said effects into a' joint effect,I and determining thetime of occurrence of an abru change in the rate of change of magnitu e of said joint effect.

14.4 The method of determining when a less than five ounces passes through a critical point, which comprises determinnig the time' of occurrence lof an abrupt change 1n the rate of change of the difference' between temperatures at different localities adjacent 15. The method 'of determining when a small mass of steel of an order of wei ht lessthan five ounces passes through a critical point, whichl comprises producing effects dependent upon the temperatures at different localities adjacent said steel within a medium adjacent said steel and through which heat transfer is effected, the rate of change of temperature at at least one of said nal to said material reflecting a simultaneous abrupt change in the rate of temperature change ef said material, the step Awhich consists in magnifying the change' in the rate of change of an effect dependent upon said rate of change of temperature of said medium.

t vice controlled by'said composite tempera- 17. Apparatus for determining a critical pointi of a mass undergoing temperature .change comprising a composite temperatureresponsive device having components disposed in the heating zone at different Vdistances from said mass, and an indicating detuie-responsive device.

18. Apparatus for determining a critical point of a mass undergoing temperature change comprising a composite temperatureresponsive device having components disposed in the heatingzone at different distances from said mass, means impedin lequalization of temperature between sai components, and anindicating device con-4 ytrolled by said composite temperature-responsive device.

19. Apparatus for determining a critical point of a mass undergoing temperature change comprising a composite temperatureresponsive device having components disposed in the heating zone atdifferent distances from said mass, a baffle impeding heat change comprisin other and differently A the combination with a heating cham transferbetween said heat components; and an indicating device controlledA by said composite temperaturensive device.4

20. Apparatus for, determining' a critical point of a mass undergoing temperature effect whose rate o change depends .upon the means for producing an rate of change oftemperature. of a medium external to said mass and reflecting the corresponding rate of change of temperature of said mass, means for magnifying the change in the ,rate of chan of said eect, and means indicating sai magnified change.-

21. ApparatusV for determining a critical point of a mass lundergoing temperature change comprising means for roducing an effect whose rate of change depends 'upon the rate'of'change'of difference of .tem erature betweenregions adjacent but at different distances from said mass, and means for indicating changes in the rate of change of said effect.

22. Apparatus for determining a critical point of a mass undergoing temperature changecomp-rising a plurality of temperature-responsive devices spaced from each other and differentl disposed with respect to said mass', where y they are to different degrees responsive to temperature changes of said mass2 and indicating means controlled by said temperature-responsive devices jointly. l

23. Apparatus ior-determining a critical point of a mass undergoing temperature change comprising a plurality of temperature-responsive devices spaced from each disposed with respect to said mass, where y they are to different degrees responsive'to temperature chan es of said mass, and a curve drawing recor er cont-rolled by said temperature-responsive devices joint y. A

24. Apparatus for determining a critical point of a. mass undergoing temperature change comprising a plurality of tempera- -ture-responsive' devices at least one of'whi'ch produces an effect dependent upon the temperature at a region adjacent said mass, a plurality of said devices producing a multiple effect dependent upon the temperature at a region adjacent said mass but differently positioned with res ect thereto, and means for indicating sai first named effect in combination with said multiple effect.

25. Heat treating apparatus com rising Iirer, of a plurality of temperature-responsive devices therein and so disposed with respect to the mass undergoing heat treatment as to impede equalization of their temperatures, and indicating means external to said heat'- ing chamber subject to the conjoint control of said temperature-responsive devices.

26. The combination with a heating chamber, of a plurality of teniperature-respon- -to said heating chamber su ject to the con'- joint control of said devices.

27. The combination with a. heating chamber, of a plurality of thermo-couples in circuit with each other, at least one cold junctemperature-responsive tion located external to said chamber, an-

other cold junction disposed in said heating chamber in spaced relation from the hot junctions, and an indicating instrumentcontrolled by said couples jointly.

28. The combination with a heating chamber, of-a composite temperature-responsive device therein and comprising a plurality of serially connected thermo-couples, at least one cold junction being disposed at a point remote from the hot junctions, another cold junction disposed Vin said chamber and f spaced from said hot junctions, and an indieating instrument controlled by said composite temperature-responsive device.

29. The combination with a heating chamber, of a composite thermo-cou le comprising at least three thermo-couples in series with each other, at least one cold junction disposed at a point remote from the hot junctions, and at least two cold junctions disposed in said chamber adjacent said hot junctions, and an indicating instrument controlled by said composite thermo-couple.

30. The combination with a heating chamber, of a plurality of thermo-couples in circuit with each other, at least one cold junetion located external to said chamber, anj other cold junction disposed in said heating chamber in spaced relation from the hot junctions, means disposed between said cold junction Within said'chamber and said hot junctions for impeding equalization of temperature between said junctions, and an indieating instrument controlled by said couples jointly. A

31.' Apparatus for determining a critical point of a mass 'undergoing temperature change comprising a plurality of thermocouples having junctions disposed in the heating zone at different distances from said mass, and an indicating device controlled by said thermo-couples.

32. Apparatus for determining a critical point of 'a mass undergoing temperature change comprising a plurality of thermocouples having junctions disposedl in the heating zone at different distances from said mass, means impeding equalization of telnperature between said junctions, and an indicating device control1ed by said thermocouples.

CHARLES A. MARTIN; 

